In satellite telecommunication systems, it is desirable for RF power amplifiers to linearly amplify RF signals in a highly efficient manner. Doherty-type amplifiers are known to achieve an efficiency advantage over standard class AB and class B amplifiers near peak power, in part, because of an instantaneous modulation of their carrier amplifier's loadline as the RF input level changes. One type of high efficiency, linear power Doherty amplifier is a three stage amplifier, two stages that are carrier amplifier networks and one stage that is a peaking amplifier network.
This three stage implementation of a Doherty amplifier desirably requires a three-way power splitter with two of the three-way splits having equal amplitudes and phases, and the third split being of equal amplitude, but with a negative ninety degree phase offset with respect to the other two splits.
In the past, the power combiner/splitter function of a power coupler has been achieved by conventional and well-known methods which include the hybrid-ring couplers, the branch line coupler, in-line power splitter, the tee-combiner/divider, and the Wilkinson combiner/divider. These couplers typically divide one input signal into sub-signals through the use of a binary division scheme. Thus, a typical combiner/divider network uses a multiplicity of couplers, each of which converts an input signal into two output signals. Accordingly, such a coupler may divide an input signal into first and second intermediate signals. Another such coupler may then divide the first intermediate signal into third and fourth intermediate signals. Another such coupler may divide the second intermediate signal into fifth and sixth intermediate signals. This process of dividing one signal into two signals continues until a sub-signal for each stage of an amplifier system is provided.
A problem exists with this binary division scheme when an unequal number of signal inputs are needed for a downstream network, as in the three inputs needed for the three-stage Doherty amplifier. This is because an input signal is first divided into two intermediate signals, but only one of the two intermediate signals need be divided to produce the three secondary signals for the three-stage Doherty amplifier. So, while three secondary signals can be produced, there will be an unequal power division between them.
One prior art power coupler for accomplishing three-way power division and combination is a modified form of the hybrid-ring coupler. This modified hybrid-ring coupler system uses distributed, quarter-wave length tuning elements to achieve a three-way, equal phase power division. Unlike the familiar hybrid ring, extra line lengths are provided so that three output ports and a composite or input port are provided in addition to two isolation ports. The device is inherently reciprocal, a signal at the composite port being split three ways, substantially one-third of the power appearing at each of the three output ports. Associated line lengths between the various ports are adjusted such that signals are in-phase at the output ports and cancel at the isolation ports so that the divided energy appears in equal phase as well as equal amplitude. The problem arises when attempting to implement this power coupler with the three-stage Doherty amplifier because this modified hybrid-ring coupler system produces three secondary signals that are in-phase. This is directly counterproductive to the proper function of the Doherty amplifier, where one secondary signal is needed that desirably lags ninety degrees out of phase relative to the other two secondary signals.
To overcome the problem of in-phase output signals, such as that seen in the modified hybrid-ring coupler system, lines of sufficient length may be added to achieve the desired phase relationships between the individual output signals. Such an arrangement requires an inefficiency of layout space which adversely affects size and weight of the power coupler. Furthermore, this arrangement increases circuit losses due to the relatively large circuit path lengths required for implementation.
Another power coupler provides three properly phased, equal amplitude output signals in response to one input signal. The physical implementation of this power coupler is a waveguide system. This system is suitable when there is unlimited implementation space. However, in applications such as satellite systems, antenna systems, and so forth, waveguide systems may not be desirable due to their inherently large physical characteristics. Furthermore, waveguide systems are less efficient which may result in undesirable circuit losses of the input signal.
Thus what is needed is a three-way phase shifting power coupler that splits or combines input signals efficiently and simply. Furthermore, what is needed is a power coupler that can split one input signal into three equal amplitude output signals, two output signals being in-phase with the input signal, and the third output signal being shifted in phase relative to the other two output signals, that can be implemented with a three stage Doherty-type amplifier.